1. Field of the Invention
The present invention relates to a magneto-optical disk unit which can perform both overwriting on a magneto-optical disk by a magnetic-field-modulation overwrite method and the recording, reading and erasing of a magneto-optical disk which requires a conventional erase process, and further relates to magnetic-field generators for use with magnetic-field-modulation overwritable magneto-optical disks.
2. Description of the Prior Art
In the last few years, magneto-optical disk units capable of recording information signals at high densities have increasingly come into practical use. The magneto-optical disk units are available in two types, one that commonly uses as a recording medium an alloy of a rare earth metal and a transition metal and requires three processes of erasing, recording, and reading, and the other that enables overwriting using a magnetic field modulation method. The above-mentioned two types of conventional magneto-optical disk units are described below in terms of recording with reference to the accompanying drawings.
FIG. 25 is an illustration showing an essential part of the prior-art magneto-optical disk unit (prior art 1) of the type that requires the three processes: erasing, recording, and reading. In the figure, reference numeral 1 denotes a magneto-optical disk, wherein a magneto-optical recording layer 3 is formed on a substrate 2 by means of a thin-film formation technique such as sputtering. An overcoat layer 4 is formed on the recording layer 3. Reference numeral 7 indicates a magnetic-field generator constituted from a magnet or an electromagnet, and reference numeral 6 designates an objective lens for concentrating laser beams 5.
The conventional magneto-optical disk unit as constructed above performs its write, erase and read operations as follows.
When the record operation starts, the magneto-optical recording layer 3 on the substrate 2 has been unified in a direction of magnetization through the erase process. First, the magneto-optical disk 1 thus arranged has a magnetic field applied thereto by the magnetic-field generator 7 and then the magneto-optical recording layer 3 is heated to a temperature exceeding the Curie temperature by a laser beam 5 condensed by the objective lens 6, so that the magnetization of the magneto-optical recording layer 3 is changed and fixed, when cooled, depending on the direction of the magnetic field. In this way, the record or write process is accomplished. The laser beam 5 is actually an on/off signal modulated by a write signal and therefore the magneto-optical recording layer 3 is magnetically inverted only at those portions that have been heated.
In order to erase the previously written data, that is, in order to unify the directions of magnetization of the magneto-optical recording layer 3, the magnetic field of the magnetic-field generator 7 is inverted and then the magneto-optical recording layer 3 is continuously irradiated with laser beams 5.
In the read operation, the magnetic-optic recording layer 3 is irradiated with a laser beam, which is considerably weak, as compared with a laser beam used during the write process. Data read is performed by making use of the fact that the Kerr angle of rotation of reflection light of the laser beam varies depending on the direction of magnetization.
In the case of the above-mentioned magneto-optical disk unit, to rewrite on it, it is necessary to erase the previously written information and thereafter perform recording. Accordingly, it takes much time to record information. To eliminate this disadvantage, overwrite techniques are now being investigated with great interest. Such techniques include a so-called magnetic-field-modulation method proposed in Japanese Patent Laid-Open Publication 60-48806 (issued in 1985). This magnetic-field-modulation method allows high-speed data recording using magnetic-field generating means (flying magnetic head) that travels above a magneto-optical disk for use as data files in a floating manner.
FIG. 26 illustrates a magneto-optical disk unit (prior art 2) using the above magnetic-field-modulation overwrite method. Like parts in FIG. 25 and FIG. 26 are indicated by the same numerals. In the figure, reference numeral 11 denotes a magnetic-field-modulation overwritable magneto-optical disk wherein a special overcoat layer 8 is further formed on the magneto-optical recording layer 3. Reference numeral 9 indicates a flying magnetic head 9 and reference numeral 10 indicates a magnetic-head current driving circuit 10 for the flying magnetic head 9. When the magneto-optical disk is still, the flying magnetic head 9 and the magneto-optical disk 11 are in contact with each other. When the magneto-optical disk is rotating, the head 9 is lifted approximately a few .mu.m from the special overcoat layer 8. Namely, a contact start/stop (CSS) method is employed. For a write operation, the magneto-optical recording layer 3 is heated to a temperature exceeding the Curie temperature by the laser beam 5 continuously fired, while a modulated magnetic field is simultaneously applied to the vicinity of the heated portions by the flying magnetic head 9. The heated magneto-optical recording layer 3 is magnetically inverted depending on the direction of the modulated magnetic field and, when it is cooled, directions of magnetization are fixed, thus recorded as information. In the case of prior art 2, the magneto-optical recording layer is heated for the rewrite operation. Therefore, even if recording tracks have information already written thereon, the previous information can be erased simultaneously with the write operation, or replaced with new information. In this way, an overwrite operation is accomplished.
However, the prior art magneto-optical disk unit (prior art 2) must use a magneto-optical disk provided with a special coating to cope with the CSS method.
Disk units of prior art 1 are being commercialized at present and those of prior art 2 capable of overwriting are under development as coming-generation devices. What matters here is that the magneto-optical disks of prior art 1, given no consideration of providing a special overcoat for CSS thereto, cannot be used in the coming-generation model of prior art 2. When they serve as data files, it is critical that information written with the preceding-generation model units (prior art 1) can be read and written with the coming-generation model units (prior art 2). Thus, it has been a desire that magneto-optical disks used for prior art 1 be also usable in the units of prior art 2.
Further, the magneto-optical disk unit of prior art 2 cannot perform a stable float-travel or traveling in a floating manner unless the surface precision of the disk is satisfactory, since the flying magnetic head 9 floats only a few .mu.m or so from the aforementioned special overcoat layer 8 when the magneto-optical disk is in a rotating state. The magneto-optical disks actually used employ a substrate molded of plastic and further the special overcoat layer 8 is applied thereto by a spin coat method. This results in that the disks so constructed are not even in an outer rim portion.
FIG. 24 shows the shape of such an outer rim portion of a disk. A molded substrate has a substrate protuberance D and a substrate burr E. The size of the substrate protuberance D is approximately 5 to 30 .mu.m, and that of the substrate burr E is approximately 5 to 40 .mu.m. Although these sizes can be reduced to some extent by devising the molding conditions and molding dies, it is very difficult to make them less than 5 .mu.m. Also, after the special overcoat layer 8 is applied, there develops an upheaval in the peripheral end of the disk. The width G of the upheaval is approximately 0.8 to 1.5 mm, and the height thereof approximately 15 to 50 .mu.m. This upheaval will take place due to the action of surface tension of the overcoat material even though the substrate is perfectly molded.
The use of the conventional flying head for such a disk would cause an unstable floating and a contact with the disk resulting in damage to the special overcoat 8 when the head comes to the peripheral end of the disk.